Multi-junction is key to record solar efficiency

PARIS – Scientists at the U.S. Naval Research Laboratory (NRL) have developed a triple-junction solar cell design, which they claim has the potential to exceed 50 percent power conversion efficiency under concentrated illumination.

The NRL researchers have been collaborating with the scientists and engineers at Imperial College in London and MicroLink Devices Inc. (Niles, Illinois) to increase the efficiency of triple-junction solar cells much further. The world record stands at 44 percent under concentrated illumination.

Work has focused on different semiconductor materials and applied band structure engineering, via strain-balanced quantum wells. This has produced a design for a multi-junction solar cell that can achieve direct band gaps ranging from 0.7 to 1.8 electron volts with materials that are all lattice-matched to an indium phosphide substrate.

"Having all lattice-matched materials with this wide range of band gaps is the key to breaking the current world record," said Robert Walters, Ph.D., NRL research physicist. "It is well-known that materials lattice-matched to InP can achieve band gaps of about 1.4 eV and below, but no ternary alloy semiconductors exist with a higher direct band-gap."

To achieve high efficiency levels, NRL researchers said they identified InAlAsSb quaternary alloys as a high band gap material layer that can be grown lattice-matched to InP. They modeled the band structure of InAlAsSb and demonstrated that this material could potentially achieve a direct band-gap as high as 1.8eV.

With this result, and using a model that includes both radiative and non-radiative recombination, the research team said it created a solar cell design that can potentially exceed 50 percent power conversion efficiency under concentrated solar illumination.

Schematic diagram of a multi-junction solar cell formed from materials lattice-matched to InP and achieving the bandgaps for maximum efficiency

NRL said it will collaborate with MicroLink and Rochester Institute of Technology (Rochester, New York) to pursue a three-year materials and device development program aimed at realizing this solar cell technology.

There is a company in Australia that is working with this technology,they are called Solar Systems Pty Ltd who are a fully owned subsiduary of Silex Systems , they are developing a triple junction cell on a virtual germanium substrate, they are looking at achieving 50% or better efficiency, the substrate they will be making these cells on is much cheaper than what Spectrolabs are using, this substrate was developed by a company called Translucent Inc in the US which is also owned by Silex, they also own Solar Systems Pty Ltd.
Solar Systems got a $2 mil grant from the Australian Solar Institute to develop these cells, there were a number of companies involved with theis new cell, Solar Systems being the lead together with Translucent Inc, IQE, and Spectrolabs as well.

The question comes to why there is no solar cell manufacturers picking up the technology and start building high efficiency solar cell. There might be more than just the story in the article. I am eager to learn more.

Triple junction cells have been made by Boeing's SpectroLab division ( CA ) for quite a few years and used in all recent spacecrafts e,g. Mars Rover. They deliver 35 to 43 % conversion efficiency as predicted in Sze's Semicondutor textbook. As the layers have to be deposited slowly by MBE the cells are more expensive ( 10x of Si xTal ). For terrestrial / commercial applications CPV ( e,g. with mirrors ) have been tried.
What, if any. is the original contribution of Inferior College of Technology in this decades old concept ? Or is this just the usual editorial bias of now UK owned EE Times ?